Prevent Fish Kills: Dissolved Oxygen

Mick RibaultPond Management

Photograph of bluegill, protect your pond from fish kills.

How to prevent fish kills in your pond due to dissolved oxygen depletion in the water.

Cory Rogers- Charleston Client Service Manager, Certified Master Pond Manager

The most common cause of fish kills in ponds is a lack of dissolved oxygen in the water. Fish require dissolved oxygen in order to survive. Dissolved oxygen comes from two sources, the atmosphere and aquatic plants. Lack of dissolved oxygen, and thus the danger of a major fish kill, is increased in the warmer summer months.

Photograph of bluegill, protect your pond from fish kills by preventing dissolved oxygen depletion.
Be knowledgeable about causes of oxygen depletion to protect your pond from fish kills.

As the warm weather heats the water, the pond becomes stratified or layered. Warm water stays at the surface level while cooler water stays below. The warm layer contains more dissolved oxygen. A lack of water circulation, especially in deep ponds, can result in fish struggling at the warm surface level to find enough dissolved oxygen to survive. Additionally, a sudden extreme turnover or mixture of the layers can lead to not enough dissolved oxygen dispersed throughout the pond. One possible cause of an extreme turnover is a thunderstorm bringing cold rain.

Additional causes of a dissolved oxygen depletion and resulting fish kills are over-fertilization and a major bloom die-off. Over-fertilization of yards and fields causes excess nutrients to wash into ponds and change the make-up of the ecosystem, risking oxygen depletion. A major bloom die-off may be caused by treating a large area of nuisance vegetation at once. Nuisance vegetation can grow rapidly in the summer. Treating nuisance vegetation so as not to cause a dissolved oxygen depletion is critical. Dying blooms will consume oxygen as the plants die. If the entire pond is treated at once, the dying plants can consume all of the oxygen in the water.

Signs of a Dissolved Oxygen Depletion

  1. The most obvious sign is fish suddenly dying.
  2. Noticing fish at the surface gulping for air.

How to Prevent Fish Kills Due to Dissolved Oxygen Depletion

  1. Install an aerator- Aerators facilitate the movement of water in the pond, from the bottom up, reducing stratification. Additionally, they increase the water’s oxygen supply.
  2. Plant native aquatic plants- Aquatic submersed plants give off oxygen, helping increase the level of dissolved oxygen in the water.
  3. Do not over-treat vegetation- Do not treat the entire pond at once when treating invasive and nuisance vegetation with aquatic herbicides.
  4. Do not over-fertilize- Be very specific about fertilizers being used. Get a soil test to identify the specific fertilizer and amounts necessary.
Photograph of pond with high prevalence of nuisance vegetation.
Pond Requiring nuisance vegetation treatment.
Photograph of pond after responsible treatment so as not to lead to fish kills.
After: Pond treated in sections so as to prevent a major bloom die-off which can result in fish kills.

The warm summer months put your pond at risk for dissolved oxygen depletion and resulting fish kills. Follow our expert tips to prevent fish kills in your system and make a practice of looking for oxygen depletion signs. Please contact us to learn how we can help protect your pond and maintain your beautiful space.

We provide pond maintenance, stormwater management, and more in Charleston, South Carolina, Raleigh, Durham, Charlotte, Wilmington, North Carolina, Tampa, Sarasota, Naples, St. Petersburg, Fort Myers, Florida, and Atlanta, Georgia.

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